Suggestion. Trikes don't tip, so I would get some soft tie's and put two on the front with tie down straps pulling forward, and two on the rear pulling back. Set the parking brake, and put a bungee around the front lever.
I would not put it in gear. If the roads get very rough, and the trike moves a bit back and forth, it will beat up the drive train (IMO).
I want to trailer my 2015 Tri Glide across country in a few months. I don't know where you would tie down the front end. Harley does make an accessory tie down but it does not fit Trikes. Any suggestions anyone?